Remove uses of QString::null

It's going to be deprecated.

Change-Id: Iec967478785d327caa846a259fb8eeff18173429
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

examples/webenginewidgets/demobrowser/browserapplication.cpp

@@ -577,7 +577,7 @@ void BrowserApplication::setPrivateBrowsing(bool privateBrowsing)
     } else {
         Q_FOREACH (BrowserMainWindow* window, mainWindows()) {
             window->tabWidget()->setProfile(QWebEngineProfile::defaultProfile());
-            window->m_lastSearch = QString::null;
+            window->m_lastSearch = QString();
             window->tabWidget()->clear();
         }
     }

examples/webenginewidgets/demobrowser/tabwidget.cpp

@@ -882,7 +882,7 @@ QByteArray TabWidget::saveState() const
         if (WebView *tab = qobject_cast<WebView*>(widget(i))) {
             tabs.append(tab->url().toString());
         } else {
-            tabs.append(QString::null);
+            tabs.append(QString());
         }
     }
     stream << tabs;